Surface-emitting laser element is a semiconductor laser element capable of emitting laser light in the direction vertical to the surface of the substrate. As the surface-emitting laser element, a GaAs-base one, for example, has ever been developed, which comprises a pair of DBR (diffractive Bragg reflector) resonators formed on a semiconductor substrate such as GaAs, and composed of an AlGaAs/AlGaAs pair differed from each other in the Al composition, and an AlGaAs-base active layer disposed between the pair of DBR reflectors and serves as a light-emitting region.
In this type of surface-emitting layer element, it is necessary to restrict a sectional area of current path of current injected into the active layer, for the purpose of raising the emission efficiency and lowering the threshold current. In recent years, as a method of restricting the current path, a method of forming a current confinement structure is becoming a mainstream, in which a high-Al-content layer, typically an AlAs layer, is interlaced in a multi-layered film composing the DBR reflector, and only a portion of the high-Al-content layer is selectively oxidized to thereby convert it into Al2O3 having a large electric resistance.
An exemplary configuration of a surface-emitting laser element having the current confinement structure obtained by oxidizing an AlAs layer will be explained referring to FIG. 4. FIG. 4 is a sectional view showing a configuration of the surface-emitting laser element.
The surface-emitting laser element 10 has, as shown in FIG. 4 and as being sequentially stacked on an n-type GaAs substrate 12, a stacked structure which comprises a lower DBR resonator 14 composed of an n-type semiconductor multi-layered film, an Al0.3Ga0.7As lower cladding layer 16, an active layer 18, an Al0.3Ga0.7As upper cladding layer 20, an upper DBR resonator 22 composed of a p-type semiconductor multi-layered film, and a p-type GaAs cap layer 24.
The lower DBR resonator 14 is configured as the semiconductor multi-layered film composed of 30.5 pairs of n-type Al0.2Ga0.8As layers and n-type Al0.9Ga0.1As layers.
The upper DBR resonator 22 is configured as the semiconductor multi-layered film composed of 25 pairs of p-type Al0.2Ga0.8As layers and p-type Al0.9Ga0.1As layers.
A compositional layer of the upper DBR resonator 22 closest to the active layer 18 is formed as a p-type AlAs layer 26 in place of the p-type Al0.9Ga0.1As layer, wherein exclusive of the center circular region thereof, the peripheral AlAs layer is converted into an Al oxidized layer 28 by selective oxidation.
The Al oxidized layer 28 functions as a high-electric-resistance current confinement region of the oxidation confinement type. On the other hand, the center circular region remains intact as the p-type AlAs layer 26, and functions as a current injection region.
The cap layer 24 and the upper DBR resonator 22 are etched to form a columnar mesa post 30 having a circular section.
On the upper surface and side faces of the mesa post 30 and on the upper cladding layer 20 on both sides thereof, an SiNx film 32 is formed.
The SiNx film 32 on the upper surface of the mesa post 30 is removed in an annular form so as to allow the n-GaAs cap layer 24 to expose therein. An AuZn electrode having an almost same annular form is formed therein as a p-side electrode 34.
On the back surface of the n-type GaAs substrate 12, an AuGe/Ni/Au film is formed as an n-side electrode 36.
In fabrication of the surface-emitting laser element 10, first a stacked structure is formed on the n-type GaAs substrate 12 by sequentially stacking the lower DBR resonator 14, lower cladding layer 16, active layer 18, upper cladding layer 20, upper DBR resonator 22, and cap layer 24. Next, an SiNx film (not shown) is formed on the cap layer 24 by the plasma CVD process, and further thereon, a photoresist film (not shown) is formed.
Next, a circular pattern is transferred onto the photoresist film by a photolithographic technique to thereby form a circular etching resist mask (not shown), and then using the resist mask, the SiNx film is etched by the reactive ion etching (RIE) process using a CF4 gas as an etching gas, to thereby form an SiNx mask.
Next, using the double-layered mask of the resist mask and SiNx mask, the cap layer 24 and upper DBR resonator 22 are etched to reach the upper cladding layer 20 by the reactive ion beam etching (RIBE) process using a chlorine gas, to thereby form the columnar mesa post 30.
Next, the stacked structure including the mesa post 30 is heated in a steam atmosphere to 350° C. to 450° C. so as to proceed oxidation until a desired diameter of oxidation confinement is obtained.
By this process, the p-type AlAs layer 26 in the upper DBR resonator 22 is oxidized selectively in a region which falls in the outer circumferential area of the mesa post 30, and thereby the Al oxidized layer 28 is formed, while leaving the center region of the mesa post 30 intact as a p-type AlAs layer 26.
Next, the double-layered mask of SiNx mask and resist mask is completely removed by the RIE process, and an SiNx film 32 is formed again over the entire surface by the plasma CVD process.
Next, the SiNx film 32 on the upper surface of the mesa post 30 is removed in an annular pattern, and the p-side electrode 34 composed of an AuZn electrode and having an almost same annular pattern is formed. Further being undergone typically through a process of forming the n-side electrode 36 composed of an AuGe/Ni/Au film on the back surface of the n-type GaAs substrate 12, the surface-emitting laser element 10 is completed.
Conventionally, the above-described steam oxidation of the AlAs layer 26 has been carried out using a steam oxidation apparatus as explained below. Next paragraphs will describe, referring to FIG. 5, a configuration of a conventional steam oxidation apparatus used for forming the oxidation-confinement-type current confinement structure in the surface-emitting laser element, by oxidizing a high-Al-content layer. FIG. 5 is a flow sheet showing a configuration of a conventional steam oxidation apparatus 40.
The steam oxidation apparatus 40 is an apparatus used for forming the current confinement structure into the surface-emitting laser element by subjecting the high-Al-content layer to steam oxidation, and is equipped with, as shown in FIG. 5, a single-slice-processing reactor 42 as a reactor for the steam oxidation.
The reactor 42 comprises a square-tube type quartz chamber 48 for housing a susceptor 46 on which a semiconductor substrate 44 comprising the surface-emitting laser element 10 having the above-described mesa post 30 formed therein is placed, and an electric furnace 50 disposed around the prism-formed quartz chamber 48.
The electric furnace 50 is a lamp heater, and can elevate temperature of the semiconductor substrate 44 through lamp irradiation.
The steam oxidation apparatus 40 further comprises a steam-accompanied inert gas system for supplying a steam-accompanied inert gas to the reactor 42, an inert gas system for supplying an inert gas to the reactor 42, a reactor bypass pipe 52 for allowing the steam-accompanied inert gas system and inert gas system to bypass the reactor, and an exhaust system for discharging exhaust gas from the reactor 42.
The exhaust system has a water-cooled trap 54, and further comprises a fourth gas pipe for introducing gases sent from a gas outlet port 42B of the reactor 42 and from the reactor bypass pipe 52 into the water-cooled trap 54, and a fifth gas pipe 58 for discharging the gas after being passed through the water-cooled trap 54.
The steam-accompanied inert gas system comprises an H2O bubbler 60 which contains pure water, and configured so as to allow the inert gas to blow and bubble therein to thereby make water vapor accompany the inert gas, a first gas pipe 64 connected to an inert gas source and sends the inert gas after being regulated by an MFC (mass flow controller) 62A to the H2O bubbler 60 so as to allow it to bubble therein, and a second gas pipe 68 for sending the steam-accompanied inert gas generated in the H2O bubbler 60 through an automatic open/close valve 66A to a gas inlet port 42A of the reactor 42.
The inert gas system is connected to the inert gas source, and has a third gas pipe 70 for sending the inert gas after being regulated by the MFC 62B in the flow rate, through the automatic open/close valve 66C to the gas inlet port 42A of the reactor 42.
The reactor bypass pipe 52 is connected, at one end thereof, to the second gas pipe 68 through the automatic open/close valve 66B and also to the third gas pipe 70 through an automatic open/close valve 66D, and at the other end thereof, to a fourth gas pipe 56 so as to allow the steam-accompanied inert gas and inert gas to bypass the reactor.
The first to fifth gas pipes are those typically made of ¼ SUS, and the automatic open/close valves 66A to 66D are disposed closer to the H2O bubbler 60 and reactor 42.
The automatic open/close valves 66A to 66D are electromagnetic diaphragm valves.
When the steam-accompanied inert gas fed from the steam-accompanied inert gas system is supplied to the reactor 42, the automatic open/close valve 66A is opened, and the automatic open/close valve 66B is closed. When the inert gas fed from the inert gas system is supplied to the reactor 42, the automatic open/close valve 66C is opened and the automatic open/close valve 66D is closed.
It is possible to send the steam-accompanied inert gas fed from the steam-accompanied inert gas system to the reactor bypass pipe 52, by closing the automatic open/close valve 66A and opening the automatic open/close valve 66B. It is also possible to send the inert gas fed from the inert gas system to the reactor bypass pipe 52, by closing the automatic open/close valve 66C and by opening the automatic open/close valve 66D.
The H2O bubbler 60 is housed in the thermostatic oven 72, and water in the H2O bubbler 60 is regulated at a predetermined temperature by the thermostatic oven 72.
A tape-formed heater 74 is wrapped around the second gas pipe 68 between the H2O bubbler 60 and reactor 42, so as to heat the second gas pipe 68 to 120° C. to 130° C. or around, to thereby suppress condensation of the steam in the steam-accompanied inert gas passed through the H2O bubbler 60.
To form the oxidation-confinement-type current confinement structure in the surface-emitting laser element 10 by oxidizing the AlAs layer 26 using the steam oxidation apparatus 40, first the inert gas in the inert gas system is supplied to the reactor 42, and the semiconductor substrate 44, that is, a stacked structure of the surface-emitting laser element 10 having the mesa post 30 formed therein, is heated to 350° C. to 450° C. or around using the electric furnace 50.
Next, in place of the inert gas, the steam-accompanied inert gas is supplied to the reactor 42, and is allowed it to stand for a predetermined duration of time.
The conventional steam oxidation apparatus 40 configured as described in the above can supply the steam-accompanied inert gas from the steam-accompanied inert gas system to the reactor 42, and can proceed the steam oxidation of the AlAs layer 26 of the surface-emitting laser element 10, while keeping temperature of the surface-emitting laser element 10 having the AlAs layer 26 thereon and the mesa post 30 formed thereon at a predetermined high temperature.
It is to be noted that the present inventors could not find any prior technical literatures disclosing configuration of the above-described steam oxidation apparatus 40, so that disclosure of information on the prior technical literatures will be omitted.
It is, however, practically difficult to wrap the tape-formed heater 74 uniformly around the gas pipes in the conventional steam oxidation apparatus 40, and non-uniform wrapping of the tape-formed heater 74 possibly occurs, for example, at a bent portion 76 of the second gas pipe 68 sometimes resulted in only an insufficient heating of that portion.
It has also been difficult to sufficiently heat the automatic open/close valves 66A, 66B using the tape-formed heater 74 due to their structures and shapes. In particular, the second gas pipe 68 in the vicinity of the gas inlet port 42A of the reactor 42 is difficult to be fully wrapped by the tape-formed heater 74, and this sometimes resulted in only an insufficient heating.
As a consequence, there was a problem in that the path of the steam-accompanied inert gas always had a portion which could not fully be heated, and this resulted in condensation of the steam in the steam-accompanied inert gas in that portion.
Any condensation of a portion of the steam in the steam-accompanied inert gas supplied to the reactor 42, in the oxidation of the high-Al-content layer of the surface-emitting laser element, may vary the amount of steam in the steam-accompanied inert gas and may consequently vary oxidation rate, and this makes it difficult to obtain the current confinement structure in a highly controllable and reproducible manner.
Although the forgoing paragraphs described the case where the AlAs layer 26 of the surface-emitting laser element 10 was treated by steam oxidation, the same problem will apply also to the steam oxidation apparatus in general in which an object-to-be-oxidized housed in the reactor is treated by steam oxidation.
It is therefore an object of the present invention to provide a steam oxidation apparatus capable of ensuring a desirable controllability and reproducibility of the steam oxidation of an object-to-be-oxidized housed in the reactor, by suppressing condensation of the steam in the steam-accompanied inert gas supplied to the reactor.